Thermoset medium-voltage insulation compounds that have been used in 5,000 to 39,000 volt transmission and distribution cable, and higher voltage cables as well, have been based on cross-linked (also referred to as cured) polyethylene (PE) homopolymers, copolymers of ethylene and alpha olefins such as propylene, butene, or octene (EP, EB, EO), and terpolymers of ethylene, propylene, and a diene (EPDM). Any of those thermoset compounds are subject to in-service polymer oxidation or ionic contamination, which can result in a breakdown of their electrical performance.
Polymer oxidation is electrochemical oxidation, and typically involves a two-step process. First, ground water often migrates through the cable jacket and the semiconductive insulation layer of the cable to contact the polymer. Second, electrical stresses at the polymer/water interface initiate a series of free-radical reactions, which lead to the decomposition of the water and the oxidation of polymer. Ionic contamination can result from (a) invasion of ionic species via ground-water ingress, as well as (b) concentration of residual ionic species in the thermoset compound.
Additives capable of reducing electrochemical oxidation by precipitation or complexation of undesirable ions may have a beneficial impact on in-service cable performance. Medium-voltage insulation compounds, which are based on either ethylene-propylene copolymer or EPDM often contain, as a preferred ingredient, lead tetraoxide, Pb3O4. Lead tetraoxide has been shown to improve wet-electrical performance in these types of cables by preventing electrochemical oxidation. However, environmental concerns about the disposal of lead-containing chemicals or materials have resulted in efforts to develop replacements for lead tetraoxide in medium voltage insulation.
There is a need, therefore, for environmentally acceptable methods and systems to reduce or eliminate electrochemical oxidation in power cables.